1. Field of the Invention
The present invention relates to a video transmission system, and in particular to a video transmission system which transmits video data through an ATM network.
As a recent communication is carried out in the form of multimedia, the transmission of multimedia information including dynamic video information has become more and more important. For the transmission of the multimedia information, the ATM network is suitable which has a high-speed transmission rate, a variable transmission rate function, and a multi-connection function.
Particularly in a video monitoring apparatus, a receiver is required with a function for receiving dynamic videos of a plurality of cameras and to display them on one screen switched over one after another.
2. Description of the Related Art
FIG. 10 shows an arrangement (1) of a prior art video transmission system, in which video data generators 1a-1n are connected to transmitters (CODEC) 2a-2n respectively, which are connected to receivers (CODEC) 4a-4n through an ATM switch 31 which forms an ATM network 3 with virtual channels VCIa-VCIn as fixed virtual connections, PVC""s (Permanent Virtual Connections (Circuits)). The receivers 4a-4n n are commonly connected to a selector 8, which is connected to a display unit 9.
In operation, the transmitters 2a-2n encode and cellulate video data received from the video data generators 1a-1n respectively and send the cellulated video information to the receivers 4a-4n respectively through the virtual channels VCIa-VCIn. The receivers 4a-4n decellulate and decode the received video information and send the decoded video data to the selector 8. The selector 8 selects one of the video data to be sent to the display unit 9, which displays the video data.
This video transmission system is defective in that the introduction cost rises high because the same number of receivers as the transmitters are required.
FIG. 11 shows an arrangement (2) of the prior art video transmission system, in which the same number of receivers as the transmitters are not required, and compared with the arrangement (1) shown in FIG. 10 where the receivers 4a-4n are provided, only one receiver 4 is provided and the selector 8 is not provided to reduce cost. Also, between one of the transmitters 2a-2n and the receiver 4, virtual channels VCIa-VCIn as switched virtual connections (SVC) corresponding to the transmitters 2a-2n are set up.
In operation, different from the arrangement (1), when the video information of the transmitter 2a is displayed on the display unit 9, the virtual channel VCIa is set up between the transmitter 2a and the receiver 4, and through the virtual channel VCIa the transmitter 2a sends the video information to the receiver 4. When the video information of the transmitter 2b is displayed, the virtual channel VCIb is newly set up between the transmitter 2b and the receiver 4, and through the virtual channel VCIb the transmitter 2b sends the video information to the receiver 4.
FIG. 12A specifically shows a system clock of the transmitters and the receiver in the arrangement (2) shown in FIG. 11.
The transmitters (CODEC) 2a-2n holds the system clock 21 respectively, and the receiver (CODEC) 4 holds a PLL circuit 51 which includes the system clock.
In operation, for example, when the transmitter 2a and the receiver 4 are mutually connected with the virtual channel VCIa, system clock standard information PCR (Program Clock Reference) which is included in the video data out of the transmitter 2a is sent to the receiver 4 through the virtual channel VCIa. In the receiver 4, the PLL circuit 51 leads in the frequency of the system clock (not shown) of its own with reference to the system clock standard information PCR so that the frequency may become the same as that of the system clock 21 of the transmitter 2a. 
FIG. 12B shows an example of the video data received by the receiver 4.
By the channel switchover operation of the ATM switch 31 the receiver 4 receives the system clock information and the video data synchronized therewith from the transmitters 2a-2e in order.
Since the system clocks of the receiver 4 and transmitters 2a-2n are asynchronous with each other, the PLL circuit 51 in the receiver 4 has to execute a lead-in operation of the clock at the time when the video data are switched over. Normally, since additional (average) processes are executed for the lead-in operation more than several tens of times for the measure of clock jitter, the lead-in time is elongated.
There are some defects in the video transmission system in the arrangement (2) as follows;
{circle around (1)} Since the video data are not sent to the receiver while the ATM switch 31 switches over the connection (the virtual channel), phenomena such as a temporary freeze or blackout in the worst case, of the display video arise;
{circle around (2)} Since more switchover time is required when the video data is sent through a plurality of ATM switches, the above phenomena arise remarkably and so are not suitable for the monitoring operation in a wide area;
{circle around (3)} The lead-in time is so long that it takes long until the system clock becomes stable.
It is accordingly an object of the present invention to provide a video transmission system comprising; transmitters in which video data from video data generators are encoded, cellulated and outputted, an ATM network which transmits the cellulated video data, a receiver in which the cellulated video data received through the ATM network are decellulated, decoded and outputted, and a display unit which displays the video data wherein at a switchover of the video data, a temporary freeze or blackout of a display screen is removed, the switchover is executed at a high-speed, and a lead-in operation time for a system clock is shortened.
[1] In order to achieve the above-mentioned object in a video transmission system according to the present invention, the ATM network provides fixed virtual connections between the transmitters and the receiver, and the receiver includes a connection control portion for designating a desired one of the virtual connections, and a header controller only extracts cells of the designated virtual connection.
FIG. 1 shows a schematic arrangement of a video transmission system according to the present invention, in which the video data generators 1a-1n which generate the video data are connected respectively to the transmitters 2a-2n, which are connected to the receiver 4 through an ATM switch 31 which forms the ATM network 3. It is assumed that between the transmitters 2a-2n and the receiver 4 fixed virtual connections PVCa, PVCb, . . . , PVCn are preliminarily set up. The receiver 4 includes the connection control portion and the header controller (both are not shown).
The connection control portion designates the virtual connection e.g. PVCa which includes the video data to be displayed from among the fixedly provided virtual connections PVCa, PVCb, . . . , PVCn. The header controller only extracts a cell VCIa of the virtual connection PVCa which is designated by the connection control portion and abandons other cells.
Namely, the video data of the video data generators 1a-1n are multi-connected by the ATM switch 31 and are inputted commonly to the receiver 4. In the receiver 4, the header controller only selects the video data of the virtual connection designated by the connection control portion.
As a result, without the switchover operation of the ATM switch 31 it becomes possible for the receiver 4 to display exclusively the video of the video data generator as designated, to avoid a temporary freeze or blackout of the display screen due to the switchover operation of the ATM switch, and to execute the switchover at a high-speed.
[2] In the above-mentioned invention [1], the receiver may further comprise a connection information memory which has stored the virtual connections to be read out in a predetermined order, and the connection control portion may read out the virtual connection stored in the connection information memory based on an externally designated connection signal and may designate the virtual connection to the header controller.
FIG. 2 shows a schematic arrangement of the receiver 4 shown in FIG. 1. An ATM physical interface 41 is connected to the ATM network (not shown), and to the header controller 42, a decellulater 43 and a decoder 44 in cascade. The header controller 42, the decellulater 43 and the decoder 44 are commonly connected to a control unit 50.
The control unit 50 includes the connection control portion 45, which is connected to the connection information memory 46, a control pattern information memory 47, a switchover timer 48, an alarm monitoring timer 49, and an external interface (not shown).
In operation, the connection information memory 46 has preliminarily stored a plurality of virtual connections in order. The external interface inputs an externally designated connection signal which designates the virtual connection by the number to the connection control portion 45. The connection control portion 45 reads out the virtual connection corresponding to the designated connection number from the connection information memory, and instructs the header controller 42 to only extract the cell of the virtual connection. The header controller 42 executes this instruction.
As a result, it becomes possible to designate the video data of the video data generator to be displayed directly from the outside.
[3] In the above-mentioned invention [2], the connection control portion may read out the virtual connection stored in the connection information memory at every external input timing instead of the virtual connection which is designated by the externally designated connection signal, and may designate it to the header controller.
This will be described referring to the schematic arrangement shown in FIG. 2. Instead of the above-mentioned externally designated connection signal which designates the virtual connection, the external interface inputs a timing signal which designates the start point of the operation to the connection control portion 45. With this input timing, the connection control portion 45 reads out the next virtual connection from the connection information memory 46, which is designated to the header controller 42.
As a result, it becomes possible to switch over a display signal which is displayed on the display unit in order according to the switchover order of the virtual connection preliminarily stored in the connection information memory at every timing which is designated from the outside.
[4] In the above-mentioned invention [3], the receiver may include inside a switchover timer which times a switchover timing of the virtual connection instead of the external input timing.
Namely, in FIG. 2 the switchover timer 48 outputs a switchover timing signal to the connection control portion 45 at predetermined time intervals. The connection control portion 45 and the header controller 42 which have received the switchover timing signal execute hereafter the same operation as in the case where they receive the above-mentioned external switchover timing signal.
As a result, it becomes possible to sequentially switch over the displayed video data according to the switchover order of the virtual connection preliminarily stored in the connection information memory 46 at the predetermined time intervals automatically by the switchover timer 48.
[5] Also in order to achieve the above-mentioned object in an image transmission system according to the present invention, the ATM network may provide a plurality of fixed virtual connections between the transmitters and the receiver, and the receiver may include a header controller which only extracts cells of a designated one of the virtual connections, a connection information memory which has stored the virtual connections to be read out in a predetermined order, a fault detector which detects a fault of the designated virtual connection, an alarm monitoring timer which times a continuous time of the fault, and a connection control portion which reads out the next virtual connection for detour from the memory when the fault continues longer than a predetermined time, and designates the virtual connection for detour as the designated virtual connection to the header controller.
Namely, between the transmitters and the receiver a plurality of e.g. fixed virtual connections (PVC1-PVCn) are set up.
The connection information memory stores the virtual connections (PVC1-PVCn) in this order. Detecting a fault in the virtual connection which is designated at present, e.g. the virtual connection (PVC1), the fault detector makes the operation of the alarm monitoring timer start.
When the fault of the virtual connection (PVC1) is timed by the alarm monitoring timer and the fault time continues longer than a predetermined time, the connection control portion reads out the next virtual connection (PVC2) for the detour from the connection information memory and designates this virtual connection (PVC2) to the header controller. The header controller only extracts cells of the virtual connection (PVC2).
As a result, by setting up a plurality of fixed virtual connections between the transmitters and the receiver, when the fault has occurred in the presently designated virtual connection (PVC1) the receiver can autonomously switch over the present virtual connection to the next one for the detour without routing again by the ATM switch.
[6] In the above-mentioned invention [5], the fault detector may detect that a stream of the video data is discontinuous.
Namely, the fault detector may detect a discontinuity of the stream of the video data as the fault of the present virtual connection.
[7] In order to achieve the above-mentioned object in a video transmission system according to the present invention, the ATM network may provide a single or a plurality of fixed virtual connections between the transmitters and the receiver, and the receiver may include a header controller which only extracts cells of a designated one of the virtual connections, a control pattern information memory for storing a control pattern, a connection information memory which has stored the virtual connections to be read out in a predetermined order, a switchover timer which times a switchover timing of the virtual connection, a fault detector which detects a fault of the designated virtual connection, an alarm monitoring timer which times a continuous time of the fault, and a connection control portion which designates, based on the control pattern, a single selected virtual connection to the header controller which corresponds to the virtual connection designated from outside, the virtual connection which is read out from the connection information memory in order at every external input timing or the switchover timing, or the virtual connection for detour which is read out from the connection information memory when the fault continues longer than a predetermined time.
Namely, in the control pattern information memory 47 shown in FIG. 2 a pattern information which designates the control pattern shown in the present inventions [1]-[6] is stored. The receiver 4 reads out the pattern information from the connection control portion 45, and executes one of the operations in the present inventions [1]-[6] based on the designated control pattern.
As a result, it becomes possible to designate the control pattern of the receiver 4 with the pattern information of the control pattern information memory 47. It is to be noted that the pattern information stored in the control pattern information memory 47 may be fixed or changed from the outside.
[8] In the above-mentioned invention [1], each of the transmitters may include a transmitter""s system clock generator, a transmitter""s clock information counter which counts the system clock information based on a clock of the clock generator, and makes the cells include the system clock information, and the receiver may comprise a connection control portion which designates one of the virtual connections at a predetermined switchover timing, a header controller which only extracts cells of the designated virtual connection, a clock information register which temporarily stores in order a system clock information value included in the extracted cells, a receiver""s system clock generator, a receiver""s clock information counter which loads the value of the clock information register at the switchover timing and counts the receiver""s clock of the clock generator, a subtracter which subtracts the values between the clock information register and the receiver""s clock information counter, and a control circuit which controls a frequency of the receiver""s system clock generator so that the subtracter value may become 0.
Namely, each transmitter makes the cell (the video data) include the system clock information whose clock of the system clock generator of its own is counted by the clock information counter and transmits the information. In the receiver, the connection control portion designates one of the virtual connections at the predetermined switchover timing. The header controller extracts the system clock information value, included in the cells of the designated virtual connection, which the clock information register stores.
The receiver""s clock information counter counts the clock of the receiver""s clock generator. In the receiver""s clock information counter, the value of the clock information register is loaded at the switchover timing. The subtracter subtracts the values between the clock information register and the receiver""s clock information counter, and the control circuit controls the frequency of the receiver""s system clock generator so that the value of the subtracter may become 0.
As a result, it becomes possible for the receiver to lead in the frequency of the system clock of its own to the frequency of the system clock of the transmitter which is connected to the presently designated virtual connection at a high-speed for the synchronization.
[9] In the above-mentioned invention [8], the system clock information may comprise a PCR clock standard information prescribed by MPEG 2.
Namely, the video information may be transmitted with the encoding method prescribed by the MPEG 2, and the system clock information may comprise the PCR (the Program Clock Reference: the clock standard information) prescribed by the MPEG 2.
[10] In the above-mentioned invention [1], the receiver may further include a video display memory for storing the video data, and the connection control portion may stop writing the video data in the video display memory when the virtual connection is designated and may freeze the present display.
Namely, the connection control portion stops writing the video information in the video display memory at the timing of designating the virtual connection.
As a result, by freezing the display video with the previous video it becomes possible to prevent the display video from its disorder due to the discontinuity of the video elementary stream at the switchover time of the virtual connection.
[11] In the above-mentioned invention [1], the receiver may further include a video decoder for decoding the decellulated video information to the video data to be sent to the video display memory and detects a stream discontinuity of the video information to generate a warning signal, and the connection control portion may mask the warning signal for a predetermined time at the switchover timing.
Namely, the video decoder detects the stream discontinuity of the video information and generates the alarm signal. The connection control portion masks this alarm signal to be neglected until a predetermined time passes from the switchover timing of the virtual connection.
As a result, this masking operation enables a stream discontinuity other than the alarm signal to be indicated without generating the alarm signal which shows the stream discontinuity due to the virtual connection switchover.
[12] In the above-mentioned invention [1], the receiver may further comprise a blackout data generator which generates blackout data for blacking out the screen of the display unit, a selector which selects the blackout data or the video data from the video decoder and outputs the video data to the video display memory, a timer which times a time until a receiving data notification signal showing that a new screen is completed is received out of the video decoder from the predetermined switchover timing as a start point and outputs a time-out signal when the time is longer than a predetermined time, and the connection control portion may instruct the selector to select the blackout data when the time-out signal is received and the video display memory to release write stop of the video data.
The above-mentioned freezing operation is generally to hold the screen before the screen is switched over to a new screen, and is released when a single screen of the video sent from a newly switched transmitter is completed. However, when there is something wrong with the new transmitter so that the new screen is not completed, the state where the screen is freezed continues. A monitoring person may not notice this abnormal state.
Therefore, in this invention [12], the timer counts the time until the reception data notification signal indicating that the new screen is completed from the switchover timing of the virtual connection as the start point is received from the video decoder and outputs the time-out signal when the time is longer than the predetermined time. The connection control portion which has received this signal instructs the selector to select the blackout data for blacking out the display screen and releases the write stop of the video data in the video display memory to cancel the freezing.
As a result, it becomes possible for the monitoring person to recognize the abnormality of the video transmission system.
[13] In the above-mentioned invention [1], the receiver may further include a cell separator, and a receiving buffer which is connected between the cell separator and the video decoder and which temporarily stores the video information from the cell separator, and the connection control portion may clear the receiving buffer at the predetermined switchover timing.
In a stable communication state, the transmitter estimates the remainder of the receiving buffer of the receiver from the remainder of the video data stored in a transmitting buffer of its own, and controls an video information generation quantity so that an overflow or an underflow failure may not occur in both of the buffers.
However, when the receiver switches over the transmitter with which the receiver communicates, the data remainder of the receiving buffer in the receiver which the new transmitter estimates does not always coincide with the actual remainder. As a result, when the receiving buffer overflows, it is necessary to throw away or clear all of the data. The time point where this overflow occurs cannot be forecast. On the other hand, when the receiving buffer underflows, it is enough to stop reading out the data from the receiving buffer and to freeze the display video.
In this invention, the connection control portion compulsorily clears the receiving buffer at the switchover timing of the virtual connection. The receiving buffer starts to store the video information of the new virtual connection switched over from the state where there is little remainder in the buffer. As a result, the receiving buffer can avoid the overflow.
[14] In the above-mentioned invention [1], the receiver may further include a superimposing portion which stores an identifier or position information of the transmitter corresponding to the virtual connection and outputs the identifier or the position information corresponding to the virtual connection designated by the connection control portion, and a multiplexer which superimposes the outputs of the superimposing portion and the video display memory by an instruction of the connection control portion.
Namely, the receiver holds a corresponding table of the virtual connection with respect to the place name where each transmitter is located or the identifier corresponding. Referring to this corresponding table, the superimposing portion outputs the place name or the identifier of the transmitter connected to the present virtual connection the connection control portion designates. The multiplexer transfers the output of the video display memory overlapped with the place name or the identifier.
As a result, the video or the place from which the video has been sent is displayed in the display unit.
[15] In the above-mentioned invention [1], the virtual connection may be designated by a virtual pass identifier or a virtual channel identifier included in the cell.
Namely, the externally designated connection signal or the virtual connection which the connection information memory stores may comprise a VPI (Virtual Path Identifier) or a VCI (Virtual Channel Identifier) of the header included in the ATM cell which the receiver receives.
[16] In another aspect, the present invention may provide an apparatus in which cellulated video data received through an ATM network providing fixed virtual connections are decellulated, decoded and outputted, comprising; a connection control portion for designating a desired one of the virtual connections, and a header controller which only extracts cells of the designated virtual connection.
[17] Furthermore, the present invention may also provide a video transmission method comprising the steps of; endoding, cellulating, and transmitting video data; receiving, decellulating, and decoding the cellulated video data received through an ATM network providing fixed virtual connections; designating a desired one of the virtual connections, and extracting only cells of the designated virtual connection.
[18] Alternatively, a video transmission method may be provided comprising the steps of; endoding, cellulating, and transmitting video data; receiving, decellulating, and decoding the cellulated video data received through an ATM network providing fixed virtual connections,; extracting only cells of a designated one of in the virtual connections; reading out the virtual connections stored in a memory in a predetermined order; detecting a fault of the designated virtual connection; timing a continuous time of the fault; reading out a next virtual connection for detour from the memory when the fault continues longer than a predetermined time; and designating the virtual connection for detour as the designated virtual connection for the extraction.
[19] Alternatively, a video transmission method may also provided comprising the steps of; endoding, cellulating, and transmitting video data; receiving, decellulating, and decoding the cellulated video data received through an ATM network providing fixed virtual connections; extracting only cells of a designated one of the virtual connections; reading out the virtual connections stored in a memory in a predetermined order; timing a switchover timing of the virtual connection; detecting a fault of the designated virtual connection; timing a continuous time of the fault; and designating, based on a stored control pattern, a single selected virtual connection for the extraction which corresponds to the virtual connection designated from outside, the virtual connection which is read out from the memory in order at every external input timing or the switchover timing, or the virtual connection for detour which is read out from the memory when the fault continues longer than a predetermined time.